Process for preparing blocks of plastics material



United States Patent 3,483,048 PROCESS FOR PREPARING BLOCKS OF PLASTICS MATERIAL Hans-Joachim Lenz and Willi Ott, Lorsbach, Taunus, Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning, Frankfurt am Main, Germany, a corporation of Germany No Drawing. Filed Oct. 19, 1965, Ser. No. 498,105 Claims priority, application Germany, Dec. 12, 1964,

Int. Cl. C23f N04 US. Cl. 1562 3 Claims ABSTRACT OF THE DISCLOSURE A process for preparing blocks of polyoxymethylene by acid etching is shown. An etching paste consisting essentially of aqueous phosphoric acid and pyrogenically prepared silicon dioxide is applied to a surface of polyoxymethylene. Etching is carried out at a temperature at which phosphoric acid is active and the etching paste is then removed. Preferably the etching paste is applied in a thickness of 0.005 to 0.01 millimeter and etching is carried out at 80 C. to 100 C. A paste consisting of 85% aqueous phosphoric acid containing 5.5% by weight silicon dioxide provides satisfactory results. An etching paste which likewise provides good results consists of aqueous sulfosalicylic acid, a non-ionic wetting agent and pyrogenically produced silicon dioxide. A third paste found to be effective consists of ammonium salts of castor oil sulphuric acid esters containing silicon dioxide in quantities sufiicient to form a paste.

The present invention relates to a blocks of plastics material.

It has already been proposed to prepare blocks of plastics material by etching a plate or another shaped article of polyoxymethylene by means of an acid. For this purpose concentrated acids are used. The use of a concentrated acid, however, requires the application of special resistant coating varnishes. Besides, concentrated acids are inconvenient to handle and, moreover, when the blocks are dipped into the etching baths their flanks and reverse sides have to be covered.

Now we have found a process for preparing blocks of polyoxymethylene by etching with an acid according to which the images to be printed are worked out by etching the block at a temperature between 40 C. and the softening point of polyoxymethylene by means of an acid which is partially applied and which has been thickened to a. paste by acidproof fillers.

By polyoxymethylenes are understood the known polymers of formaldehyde or trioxane and the copolymers of formaldehyde or trioxane which contain up to by weight of cyclic ethers or formals, for example, diglycol formal, dioxolane, butane diol formal, tetrahydrofurane or ethylene oxide.

The desired images to be printed are first covered in known manner with acidproof varnishes. With these varnishes drawings can be made directly on the block material. Other known methods of transmission, for example, photomechanical transmission, may also be applied. According to the desired depth of etching the etching may be carried out in one or several stages.

As acids may be used inorganic acids such as phosphoric acid, organic acids such as sulfosalicylic acid or compounds that react as acids at the elevated reaction temperatures only, for example, the ammonium salts of castor oil sulfuric acid esters (Turkey-red oil) which at an elevated temperature act as acids while splitting oif ammonia. Solid acids are dissolved in water or mixtures of process for preparing 3,483,048 Patented Dec. 9, 1969 water and solvents that are miscible with water before the thickening fillers are added. They are preferably dissolved in the presence of wetting agents. As wetting agents which bring about a better wetting of the surface there may be used acidproof non-ionic products, for example, reaction products of tributyl phenol and ethylene oxide. Suitable fillers are acidproof inorganic fillers, for example, titanium dioxide, barium sulfate or chromium-III- oxide, pyrogenically prepared silicon dioxide being particularly suitable. I

The etching pastes according to the invention, for example, phosphoric acid of strength, are thickened with about 5% of pyrogenically produced silicon dioxide and may be applied to the polyoxymethylene plates by means of a brush or spatula. The etchings obtained are very exact since even at an elevated temperature 'the thickened pastes do not spread in an undesired way but penetrate vertically into the parts of plastics material to which they have been applied, thus etching out the images to be printed. The etching pastes are caused to act at a temperature within the range of 40 C. to the melting point of the polymer, preferably of 70 C. to 150 C. Due to the application of heat the etching times are within the range of 5 to minutes, preferably 10 to 30 minutes, depending on the depth of etching.

By the addition of a more or less large amount of filler the paste may be brought to the consistency which is most favorable with regard to the kind of its application. Since the etching is carried out at an elevated temperature less aggressive acids which consequently can be handled more easily may be used.

When etching pastes are used they cover both the bottom and the flanks of the image that has been worked out, the cross-section of the image thus having the form of a U. In the usual metal etching processes this can only be brought about by means of the so-called flank protection. Another advantage of the etching paste is that a previous etching can, if necessary, be corrected by etching certain parts of a block again. In such a case it is not necessary to cover the block additionally as has to be done when liquid acids are used. Particularly suitable for this purpose are the ammonium salts of castor oil sulfuric acid esters which owing to their neutral character can be handled particularly well at' room temperature. A special advantage of the etching pastes based on sulfosalicylic acid is that even at room temperature these pastes attack the known copying layers based on polyvinyl alcohol to such a slight extent only that the covered parts are not etched.

The blocks which have thus been prepared have an excellent hardness, a good toughness, a high scratch resistance and a low degree of abrasion. Their dimensional stability is independent of the atmospheric moisture. Owing to their flexibility thin plates may be laid around cylinders as is usually done in flexography. The hardness of the material enables very sharp prints to be obtained.

The etching process also enables very fine lines or characters to be transmitted. The resistance to solvents is very good. Blocks of polyoxymethylene or copolymers of trioxane can practically be used for all printing inks.

The following examples serve to illustrate the invention but they are not intended to limit it thereto.

EXAMPLE 1 An etching paste consisting of phosphoric acid of 85% strength and containing 5.5% of pyrogenically prepared silicon dioxide was applied by rolling to an extruded plate of polyoxymethylene which had a thickness of 1.75 millimeters and which at the places which were intended to produce the print was covered with an acidproof varnish. The layer of etching paste applied had a thickness within the range of about 0.005 to 0.01 millimeter. Etching was carried out for minutes at a temperature within the range of 80 C. to 100 C. After washing with water and removal of the coating varnish a block having neat contours was obtained.

EXAMPLE 2 4.5 parts of sulfosalicylic acid,

65 parts of water,

21.5 parts of a non-ionic wetting agent, the reaction product of 1 mol of tributyl phenol and 8 mols of ethylene oxide,

8.2 parts of pyrogenically produced silicon dioxide.

The etching paste was caused to react at 100 C. for a period of 15 to minutes. After cooling, the remaining portions of the etching paste and the coating were removed with water. An autotypy having sharp contours and a depth of etching of 0.005 to 0.1 millimeter was obtained.

EXAMPLE 3 In the manner described in Example 2 a line etching was produced on an injection-molded plate, 1 mm. thick, of polyoxymethylene. In order to increase the depth of relatively large parts of the plate which lay free and which were not intended to produce prints an etching paste prepared from ammonium salts of castor oil sulfuric acid esters (Turkey-red oil) and containing 7% of pyrogenically produced silicon dioxide was applied by means of a brush. The etching paste was caused to react at 150 C. Within 15 minutes an etching depth of about 0.05 millimeter and after minutes an etching depth of 0.1 millimeter was obtained. After cooling, the plate was washed with methyl alcohol.

We claim:

1. A process for preparing thin printing plates of polyoxymethylene by acid etching comprising the steps of: applying, by screen printing technique, 0.005 to 0.01 millimeter of an etching paste consisting essentially of aqueous phosphoric acid with 5.5 percent by weight pyrogenically prepared silicon dioxide to a surface of said polyoxymethylene; etching said polyoxymethylene therewith at a temperature of 80 C. to C.; and removing said etching paste from etched polyoxymethylene.

2. A process for preparing thin printing plates of polyoxymethylene by acid etching comprising the steps of: applying, by screen printing techniques, an etching paste consisting essentially of 4.5 parts Water, 65 parts sulfosalicylic acid, 21.5 parts of the reaction product of one mole of tributyl phenol and 8 moles of ethylene oxide, and 8.2 parts of pyrogenically produced silicon dioxide to a surface of polyoxymethylene; etching said polyoxymethylene therewith at 100 C. for 15 to 30 minutes; and removing said etching paste from etched polyoxyrnethylene.

3. A process for preparing thin printing plates of polyoxymethylene by acid etching comprising the steps of: applying an etching paste consisting essentially of 93% of at least one ammonium salt of a castor oil sulfuric acid ester and 7% of pyrogenically produced silicon dioxide to a surface of polyoxymethylene; etching said polyoxymethylene therewith at C.; and removing said etching paste from etched polyoxymethylene.

References Cited UNITED STATES PATENTS 3,235,426 2/ 1966 Bruner 1562 3,248,271 4/ 1966 Reilly et a1 156--2 3,255,060 6/1966 Neumann et al. 1562 JACOB H. STEINBERG, Primary Examiner UJS- Cl. X.R. 15616; 25279.4 

